This invention relates to halftoning, and more particularly, to halftoning in a hyperacuity printer.
Various optical and electronic techniques have been proposed for transforming continuous tone and other types of variably shaded monochromatic and polychromatic images (collectively referred to herein as "toneart images") into halftone images. To simplify this disclosure, substantial portions of the following discussion focus on monochromatic halftoning, but it is to be understood that the same general teachings apply to the halftoning of the color separations of polychromatic images. The halftoning of black and white "grayscale" images is a convenient example of monochromatic halftoning, so it is noted that the term "grayscale" is used herein as a generic descriptor for the tones that can be produced by mixing any two reference colors together in any desired proportions.
As is known, a halftone image is a binary image that is composed by writing "halftone dots" into a spatially periodic, two dimensional, tiled array of dimensionally identical "halftone cells." These halftone cells spatially correspond in the halftoned output image to respective small, spatially distinct areas of the source image. Furthermore, the surface area of the dot that is written into each of the halftone cells is modulated in accordance with a suitable measure of the perceived or "average" grayscale level of the spatially corresponding area of the source image (this "dot area" parameter typically is expressed by referring to the percentage of the halftone cell area that is filled by the halftone dot that is written therein). Thus, halftoning imparts an illusion of shading to the halftoned image because the halftone dots are written at a spatial frequency (usually called the "screen frequency") that exceeds the cyclical acuity of the human eye at normal viewing distances (a conservative rule of thumb is that the human eye is insensitive to cyclical contrast variations that occur at a spatial frequency in excess of about 40 cycles per degree within the field of view).
As will be appreciated, halftoning is an important tool for preserving the shaded appearance of toneart images that are printed using binary printing technologies. Optically screened lithographic halftoning processes have established a challenging benchmark for the imaging fidelity that can be achieved by printing halftoned images on high gamma, photosensitive recording media (i.e., a recording medium having a steeply sloped exposure vs. contrast characteristic). Modern xerographic printers utilize high gamma photoreceptors to print lineart images that approach the fidelity of lithographically printed lineart. Heretofore, however, the electronic halftoning processes that have been available for use in xerographic printers have not had sufficient imaging fidelity to enable xerographic printers to effectively compete with lithographic printing processes in the printing of high fidelity halftoned images. Xerographic printing has made significant inroads into the lithographic printing market because of its cost advantage and the improvements in its imaging fidelity. This trend is expected to continue, but it clearly will be necessary to further improve the fidelity of electronic halftoning for xerographic printers to become a fully acceptable alternative to lithographic printers for the printing of halftoned images.
Some workers have proposed electronic halftoning techniques that more or less directly emulate angularly oriented optical halftone screening functions. See, for example, Perriman et al. U.S. Pat. No. 3,997,911, which issued Dec. 14, 1976. Others have focused on modulating the size of the halftone dots that are written into tiled arrays of electronically generated halftone cells at a selected screen angle. See, for example, Hell et al. U.S. Pat. No. 3,688,033, which issued Aug. 29, 1972 and Gall et al. U.S. Pat. No. 4,499,489, which issued Feb. 12, 1985. This invention builds on these electronic halftone generators with their x/y addressable table look-up memories for tracking the scan spot as it scans across each of the electronically generated halftone cells at the selected screen angle.